Its possible to get past SOSUS, if they have inside information, they have the expertise to cloak a sub with “biologicals” noise, in effect make it sound like a whale.
In reality if it had happened we would have seen a pretty hard and fast response by our Naval resources, which did NOT happen.
I am thinking that we shot off something that was aimed at something else, possibly a satellite in a degraded and dangerous re-entry path, maybe an obsolete bird with plutonium aboard, or an old spy bird.
SOSUS has been typically deployed at geographic "chokepoints," such as in the Greenland-Iceland-UK gap in the Atlantic. The Pacific Ocean has far fewer geographic gaps, and therefore much more challenging to deploy. That said, I would suspect that near major submarine bases, such as San Diego and Bremmerton, Washington, some type of surveillence is in place.
Given that this launch was closer to LA (I believe), there may be fewer options.
Furthermore, it's not likely a submarine could "cloak" itself with biological noise. The loudest machinery would still be detectable, and its signal-to-noise ratio would still be present. If a sub needed that much biological noise, you could track the bio-noise alone! If the sub were quiet, you wouldn't waste time cloaking it, you'd just transit at your quietest speed and be done with it.
To the point about a quick and decisive response by us--maybe, maybe not. If a foreign sub did get that close, and then shot off a missile, that would be considered incredibly embarrassing for us. As long as there was no imminent threat to the United States, why bring further attention to it? It's quite possible that if there were a US sub in the area, it may have been tasked to track the invading sub to learn more about it.
In fact, it's quite possible that if we knew about a planned launch, we may have allowed it, just so we could monitor it.